CURRENT STATE AND PROSPECTS FOR THE DEVELOPMENT OF BULLET TRAPPERS FOR SHOOTING RIFLED FIREARMS..
Petrenko Evgeny Sergeevich
CURRENT STATE AND PROSPECTS FOR THE DEVELOPMENT OF BULLET TRAPPERS FOR SHOOTING RIFLED FIREARMS.
Source: Magazine «Special Equipment», 2000, No. 6.
Conducting ballistic examinations and creating bullet-and-cartridge libraries for rifled firearms is associated with the use of special devices for the non-destructive capture of bullets — the so-called bullet traps.
The circuit solutions of bullet catchers are determined by the requirements of performing work in indoor conditions with sufficiently high levels of productivity and service life of the braking medium. Various liquids, bulk, porous and fibrous materials are used as braking medium.
The first models of bullet traps were containers with water or oil. The weapon was fired vertically downwards or at some angle to the liquid surface. The main disadvantage of such bullet traps is the high level of dynamic loads on the bullets at the initial stage of the bullet's penetration into the liquid medium, leading to partial or complete destruction of expansive, jacketless and semi-jacketed bullets and significant deformations of jacketed bullets, especially at speeds over 500 — 600 m/s. The level of such loads is determined mainly by the value of the square of the bullet's speed, its transverse load (the ratio of the bullet's mass to its cross-sectional area) and the initial density of the braking medium r o. For the liquids used in bullet traps, the initial density ranges from r o = 1 g/cm3 for water to r o = 0.8 — 600 m/s. 0.9 g/cm3 for machine oils.
The low compressibility of these liquids, and above all water, and the high level of pressure arising at the boundary of the penetrating bullet and the liquid, causes the development of a hydraulic shock process capable of destroying the bullet trap container even without direct contact of the bullet with the walls of the container.
When firing a weapon at an angle to the liquid mirror, as a rule, the bullet deviates from a straight trajectory, which is especially typical for 5.45 mm caliber bullets with a displaced center of gravity, with possible subsequent contact with the walls of the bullet trap container. As a result of such contact, the bullet can be deformed, and the wall of the container can receive a through hole. When firing a weapon at a normal to the liquid mirror, certain difficulties are created by the limited ceiling height of the premises and the need to use complex designs of weapon holders for working with old and homemade weapons, for which there is a high probability of spontaneous destruction during the shot. In addition, certain restrictions are imposed by the requirements for ballistic examinations.
Thus, bullet traps using liquids as a braking medium have a number of disadvantages that limit the scope of their use.
To catch shell bullets, a braking medium made in the form of a sequence of rubber plates with r o = 1.2 — 1.8 g/cm3 can be used. The disadvantage of bullet catchers built on this principle is both the high level of dynamic loads on the bullets, exceeding the corresponding indicator for bullet catchers with a braking medium based on liquids, and a low service life due to the effect of substance carryover and amounting to several dozen to several hundred shots. In some cases, this resource is sufficient for firing a small batch of weapons. At the same time, to facilitate the extraction of bullets from the braking medium in the bullet catcher, plates are used, the thickness of which is commensurate with the length of the bullet.
The use of materials with a lower r o value as braking media compared to rubber, water and machine oil helps to increase the preservation of traces of weapons on bullets when they are fired. Such materials include porous rubber, polyurethane foam, foam plastic, etc., with a value of r o = 0.3 — 1.0 g/cm3. With a significant decrease in the level of dynamic load on the bullets, the problem of low service life remains unresolved. In addition, in the case of catching high-speed bullets with an initial velocity of 900 m/s or more (Mosin rifles, SVD), intense heating of the braking medium with a melting point of ~ 80 — 150 ° C, in particular foam plastic, can lead to its sintering on the surface of the bullet with the formation of a crust. Removing such a crust can damage traces of weapons on the bullet.
Photo 1. Bullet catcher PU-1R
In general, reducing the density of the braking medium due to the use of porous materials increases the preservation of traces of weapons on the caught bullets, however, does not solve the problem of non-destructive catching of expansive and unjacketed bullets.
Further development of bullet catchers is associated with the use of fibrous materials with a melting point of at least 500 ° C. The optimal option is to use a braking medium made of ballistic-resistant materials with a melting point of ~ 600 ° C, which include ultra-high modulus fiber (UHM) and Kevlar. Moreover, UHM surpasses Kevlar in this indicator.
The first examples of bullet catchers with a fibrous braking medium used a continuous thread of SVM or Kevlar in the form of a tangle obtained by removing the thread directly from the spool without additional processing. The density of the braking medium is 0.05 — 0.07 g/cm3. To prevent bullets from flying out of the braking medium, it is placed in a block in steel (3 — 4 mm thick) or fiberglass (8 — 15 mm thick) pipes with an internal diameter of 250 — 315 mm. The length of the braking path of bullets is:
| for pistols of caliber 6.35 — 1.5 mm. 9 mm | — | 150 … 250 mm; |
| for systems such as AK-74 caliber 5.45 mm | — | 300 … 700 mm; |
| for AKM type systems of 7.62 mm caliber | — | 450 … 1200 mm; |
| for rifles and carbines of the SVD type | — | 600 … 1500 mm. |
When using fibrous braking media, the main braking effect is created by sequentially involving in the movement of initially stationary sections of the thread, the speed of which increases from 0 to a certain value V ? V p , where V is the current value of the thread section speed, V p is the current value of the bullet speed. Neglecting the second-order smallness values, the dependence for calculating the current value of the braking force T(t) can be written as follows:
| where | l | — | linear mass of thread; |
| Vi(t) | — | current speed value of the i-th thread section; | |
| Li | — | length of the i-th thread section ; | |
 |
— | acceleration of the i-th thread section; | |
| Cxb | — | aerodynamic drag coefficient of the lateral surface of the threads in air; | |
| r in | — | air density; | |
| Fбi | — | lateral surface area of the i-th section of the thread. |
As can be seen from this dependence and the numerical calculations, the braking force T(t) in the process of bullet braking begins to increase smoothly from 0 at the beginning of the process of bullet penetration into the fibrous medium, when the mass of the thread sections involved in the movement is negligible, to Tmax in the middle section of the trajectory at Vп = (0.4 — 0.6) Vпо, where Vпо is the initial velocity of the bullet. In this case, the absolute value of Tmax is 3 — 10 times less than the corresponding parameter for a similar bullet at the initial stage of its penetration into liquid and porous media. This ensures the possibility of non-destructive trapping of many types of bullets, including expansive and jacketless ones, having an initial velocity of up to 500 — 1000 m. 600 m/s.
The service life of the braking unit is on average about 10,000 weapon firing cycles.
In more advanced models of bullet catchers, in particular, the PU-1Mu type, developed and manufactured in Russia, the optimized braking medium is a set of individual SVM fibers 5 — 20 calibers of the fired weapon. There are a number of patents for inventions for this design of the bullet catcher and the composition of the braking medium.
Due to the principle of constructing the braking medium from a set of individual fibers not connected to each other, additional possibilities of influencing the value of T are provided, in particular, reducing the absolute value of Tmax and, accordingly, creating conditions for smoother braking of bullets. This makes it possible to use bullet catchers of the PU-1Mu type for non-destructive catching of all known types of bullets, including expansive ones, having an initial velocity of up to 1100 — 1200 m/s.
This effect is primarily due to the lack of a «rigid» adhesion of the thread sections to each other, unlike the variant with one continuous thread. The interaction of individual threads with each other during the braking of a bullet is carried out mainly due to friction forces. The nature of such interaction in dependence (1) for calculating the braking force T is taken into account using the interaction coefficient kв, the value of which lies in the range from 0 to 1:
where l i is the linear mass of the i-th thread (in the general case, sections of different threads are used).
At kв = 1, the braking composition is made of one continuous thread, when the involvement of one of the sections of the thread in the movement causes the unconditional involvement of the neighboring sections of the thread in the movement, including outside the projection of the bullet. In the complete absence of interaction between individual threads, when in the limit the friction coefficient tends to 0, kв = 0. In real conditions 0<kв<1. The specific value of the coefficient kв is determined by the local density of the braking medium r o, the percentage ratio of the content of individual threads of different lengths in it, their linear mass, elasticity and friction coefficient. Thus, as a result of mechanical and thermochemical treatment of the fibrous braking medium, conditions are provided for controlling the braking force T and, accordingly, for reducing the level of dynamic load on the bullets during their braking.
A qualitative picture of the change in the braking force T for different types of bullet catchers is shown in Fig. 1.
L – the current value of the braking distance;
T – current value of braking force;
1 – for liquid and porous braking media;
2 – for fibrous braking media based on continuous thread;
3 – for fibrous braking media based on cut and processed SVM threads.
Fig. 1.
It should be noted that the area under the curves (1 – 3) for similar bullets is the same, which is due to the equivalent energy parameters of the braking work.
The specific design of the PU-1Mu bullet trap (photo 2) uses cut and processed sections of SVM fiber 5 — 20 calibers of the fired weapon long, placed as a block in a cylindrical steel housing 1400 mm long and 315 mm in diameter. The density of the braking medium is 0.02 — 0.07 g/cm3. At the same time, for braking high-speed expanding bullets, the braking medium is recommended to be located along the length of the housing with a variable density, in particular, with a lower one in the area of the inlet opening with a gradual increase towards the rear wall. The walls of the bullet trap are equipped with a rubber coating, which ensures that in the event of a shot being fired at an angle to the axis of the bullet trap, the bullet ricochets with a level of residual deformation acceptable for further examination. Due to the reduction in the level of braking force T, the service life of the braking composition increases in comparison with the variant using a continuous SVM thread by 3-4 times, reaching 40,000 cycles of firing the weapon and thereby reducing the cost of implementing a single cycle. In the case of predominantly firing a weapon with an initial bullet velocity of up to 600-700 m/s from a range of 0.7-1.0 m, when the impact of high-temperature powder gases and mineralized particles on the braking medium is reduced to a minimum, the service life of one block of the braking medium increases to 70,000-7000 cycles. 80,000 shots.
Photo 2. Bullet trap PU-1Mu
Thus, the use of cut and processed sections of SVM fiber 5 — 20 calibers of the fired weapon as a braking medium of the bullet trap allows for non-destructive trapping of 5.45 — 12.7 mm caliber bullets with an initial velocity of up to 1100 — 1200 m/s.